This invention relates to the control of an internal combustion engine in response to combustion pressure and particularly to the control of engine ignition timing for the reduction of engine knock.
The development of piezoelectric pressure sensors suitable for combustion pressure sensing and microcomputers suitable for internal engine control has created a great deal of interest in engine control in response to combustion pressure, since it is recognized that a great deal of useful information about the combustion process is derivable from the combustion pressure curve.
One approach to the analysis of a combustion pressure curve is the examination of its harmonics. An approximation of a typical combustion pressure waveform and its harmonics at the engine firing frequency, twice this frequency and four times this frequency are shown FIGS. 5b, 5c, 5d and 5e, respectively. I have certain relationships between these harmonics that show promise in engine control. One such relationship involves the amplitude ratio of corresponding peaks of two different harmonics of the combustion pressure curve. The amplitude ratio of a higher harmonic such as the fourth, at four times the fundamental frequency, to the first harmonic, at the fundamental frequency, tends to decrease with a slower burn and increase with a faster burn or with knock. In my U.S. Pat. No. 4,699,107, issued Oct. 13, 1987, I described an engine dilution control based on this amplitude ratio. In my U.S. Pat. No. 4,699,105, also issued on Oct. 13, 1987, I disclosed the control of engine ignition timing for MBT in response to the phase difference between corresponding peaks of two harmonics in the combustion pressure waveform. In this patent, I deal with ignition timing retard to prevent knock in response to the amplitude ratio of two harmonics in the combustion pressure waveform.